Just, that. Don't care how or why. AU childhood? Mission gone horribly wrong? So long as the slavery is real enough that Luke is glad to be rescued from it, even by Vader.

Bonus for lots of gratuitous violence against slavers and Hutts.

Double bonus for Luke being helpless and vulnerable, possibly in ways that involve gold bikinis.

Ultra bonus for Aphra being an advanced scout who finds him finds him first and then calls in her boss. Or, really, any Aphra at all.